The idea of prefabricating things
for convenience and speed has been with us for centuries. I can almost
see some ancient craftsman saying "I should just make a ton of these little
annoying pieces now, and just use them as I need them" to himself.
Think of a prefab as a building block with a custom shape.

Using a prefab in level editing provides
a world of construction unto itself. Besides making things like light
fixtures and walls, you can make entire rooms and level sections.
An example of this is the testroom I use to build prefabs. It consists
of a goodly sized room and a PLAYER_START. When I want to see my
progress or test the entities on a prefab I'm building, I just pop the
testroom in and compile. After I'm done, I can just delete the testroom
in one fell swoop.

Using prefabs for subtraction is another
unique aspect. I have a set of generic groups of brushes that I can
use for cliffs and chasms. I'm working on another set for windows.
You could even make a set of prefab chisels if you wanted to. The
possibilities for subtraction are just about as vast as solid prefabs.

Different editors have different methods
of getting prefabs though. Worldcraft, for example has a prefab interface
while Qoole uses a directory named prefab that can be managed with Explorer.
Check in the help section of whatever editor your using if you have problems.

Some Tricks and Hints:

If your editor has a "Texture Lock"
capability, use it when you move your prefabs around.

Always check the entities that make
up the prefab once you have it in place. I usually forget to change
the name of some trigger as I'm making a map. Pay special attention
to TEAM values when adding doors (though seeing all of the doors on a level
open at once is kind of neat).

Remember that anything can be a prefab.
If your sick of placing a small health next to a large one a million times,
make it a prefab.

If your editor lets you group
your prefabs into categories, make one for whatever map you're
working on. You can do this with some of them by just
making a new directory in your "prefab" directory. I usually
do this to get an idea going. As I'm thinking of the feel
of a level, I'll make a few objects and save them there.
It really helps to give a level a consistent feel.